fix: bad prometheus schema for headers

[zhongwencool]

Fixes

1. default headers should be a empty map, not a empty list.
2. Plain-text should be displayed with priority, json will be deprecated soon.

Summary

🤖 Generated by Copilot at 75cad5e

This pull request fixes a bug in the prometheus api openapi spec, simplifies the headers config for the prometheus push gateway, and updates the emqx_prometheus version number.

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• Added tests for the changes
• Changed lines covered in coverage report
• Change log has been added to changes/(ce|ee)/(feat|perf|fix)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• If there should be document changes, a PR to emqx-docs.git is sent, or a jira ticket is created to follow up
• Schema changes are backward compatible

Checklist for CI (.github/workflows) changes

• If changed package build workflow, pass this action (manual trigger)
• Change log has been added to changes/ dir for user-facing artifacts update